1. Field of the Invention
The invention relates to a magnetic-inductive flow meter with a plurality of functional units and with a housing which is common to all of the functional units, each functional unit having at least one measuring tube for the throughflow of an electrically conductive medium, a magnetic field generating apparatus for generating a magnetic field which runs at least perpendicular to the longitudinal axis of the measuring tube, and at least two measuring electrodes, a virtual connecting line of the two measuring electrodes running at least essentially perpendicular to the direction of the magnetic field which is permeating the measuring tube perpendicular to the longitudinal axis of the measuring tube.
2. Description of Related Art
Magnetic-inductive flow meters have been widely known in the prior art for decades. Reference is made by way of example to the literature citation Technical Flow Rate Measurement by Dr. Eng. K. W. Bonfig, 3rd edition, Vulkan-Verlag Essen, 2002, pp. 123 to 167 and also to the literature citation Principles of Magnetic-Inductive Flow Rate Measurement by Cert. Eng. Friedrich Hoffmann, 3rd ed., 2003, publication of the company KROHNE Messtechnik GmbH & Co. KG.
The basic principle of a magnetic-inductive flow meter for measuring the flow rate of a flowing medium goes back to Michael Faraday who suggested the use of the principle of electromagnetic induction for measuring the flow velocity of an electrically conductive medium.
According to the Faraday Induction Law, in a flowing, electrically conductive medium which is permeated by a magnetic field, an electrical field intensity arises perpendicular to the flow direction of the medium and perpendicular to the magnetic field. The Faraday Induction Law is used in magnetic-inductive flow meters in that, by means of a magnetic field generating apparatus which has at least one magnetic field coil, conventionally, two magnetic field coils, a magnetic field is generated which changes over time during a measurement process and the magnetic field at least partially permeates the electrically conductive medium which is flowing through a measuring tube. In doing so, the generated magnetic field has at least one component perpendicular to the longitudinal axis of the measuring tube and perpendicular to the flow direction of the medium.
By the statement in the introduction that each functional unit has a magnetic field generating apparatus for generating a magnetic field which runs at least also perpendicular to the longitudinal axis of the measuring tube, it is pointed out that the magnetic field does preferably run perpendicular to the longitudinal axis of the measuring tube or perpendicular to the flow direction of the medium, however it is sufficient that one component of the magnetic field runs perpendicular to the longitudinal axis of the measuring tube or perpendicular to the flow direction of the medium.
It was stated at the beginning that each functional unit includes at least two measuring electrodes, the virtual connecting line of the two measuring electrodes running at least essentially perpendicular to the direction of the magnetic field which is permeating the measuring tube. Preferably, the virtual connecting line of the two measuring electrodes, in fact, runs more or less perpendicular to the direction of the magnetic field which permeates the measuring tube.
The electrical field intensity which is produced by induction in the flowing, electrically conductive medium can be measured as electrical voltage by measuring electrodes which are directly, electrically in contact with the medium or also can be capacitively detected by electrodes which are not directly electrically in contact with the medium. Then, the flow rate of the flowing medium through the measuring tube is derived from the measured voltage.
The measurement error in the magnetic-inductive flow meters known from the prior art is relatively small today; a measurement error less than 0.2% can be accomplished.
Examples of known magnetic-inductive flow meters are German patent disclosure documents 197 08 857, 10 2004 063 617 (which corresponds to U.S. Pat. No. 7,261,001 B2), 10 2008 057 755 (which corresponds to U.S. Pat. No. 8,286,503 B2) and 10 2008 057 756 (which corresponds to U.S. Pat. No. 8,286,502 B2) which are hereby incorporated by reference into this patent application.
In a host of applications, it is necessary to arrange and operate several magnetic-inductive flow meters adjacent to one another.
For the following considerations, a first and a second magnetic-inductive flow meter are adjacent when at least the magnetic field which has been generated by the magnetic field generating apparatus of the first flow meter at least partially permeates the measuring tube of the second flow meter. Of course, an adjacent arrangement is not limited to two flow meters.
Often it is not possible, for example, under limited conditions of space, to choose the 3-dimensional distance of magnetic-inductive flow meters to be so great that they are not adjacent, i.e., are far enough apart that the field generated by one does not affect the other. Shielding of the flow meters would be associated with additional costs and effort.
If the first flow meter and the second flow meter in operation are carrying out measurement processes, on the one hand, it is unknown whether the measurement processes of the two adjacent flow meters are overlapping in time, and on the other hand, in the case of time overlapping, it is unknown how great the generally inconstant time overlapping is.
If time overlapping of the measurement processes of the two adjacent flow meters is assumed, in the measuring tube of the second flow meter, the magnetic field which has been generated by the magnetic field generating apparatus of the second flow meter and the magnetic field which has been generated by the magnetic field generating apparatus of the first flow meter and which extends to the measuring tube of the second flow meter are superimposed. The superposition of the two magnetic fields results in an induced electrical voltage which varies in an unknown manner and a corresponding influence on the flow rate measurements; this means a reduction of the measurement quality. Thus, for example, at a constant flow through the measuring tube of a flow meter a varying flow rate can be displayed by the flow meter. Of course, the measurement process of the second flow meter also influences the measured value of the flow rate of the first flow meter.
German patent application 10 2011 112 763.5 and corresponding U.S. Patent Application Publication 2013/0061685 A1 (which are not prior art) relate to the problem of improving the measurement quality for adjacent magnetic-inductive flow meters and an improved arrangement of adjacent magnetic-inductive flow meters. The process of these disclosures teaches the synchronization of measurement processes of individual adjacent flow meters to prevent variations of mutual influences on the flow rate measurements by the magnetic fields of adjacent magnetic-inductive flow meters.
According to what was stated in the introduction, here, it is not a matter of three-dimensionally separate, but adjacently located magnetic-inductive flow meters, but rather a magnetic-inductive flow meter with a plurality of functional units and with a housing which is common to all functional units. It is therefore a matter of several magnetic-inductive flow meters which are not 3-dimensionally separated to the extent that they have a common housing. This flow meter is produced and marketed by the company Kirchgaesser Industrie Elektronik GmbH under the name “MULTIMIDEX”.